Sodium cotransport systems in epithelial secretion.

1. After considering the direct coupling and indirect coupling to the sodium gradient in sodium-dependent secretion across epithelia, the properties of the Na-K-Cl cotransporter involved in active chloride secretion and active chloride absorption are summarized. 2. A comparison between cellular mechanism of secretion and absorption shows that the direction of transepithelial transport is determined mainly by the intracellular localization of sodium cotransport systems and sodium-independent leaks. 3. Sodium cotransport systems, performing a similar function in various epithelia or species, may provide a powerful model to study their function at a molecular level.